Author Topic: Grinding a 10" F/4 mirror for a first project  (Read 237 times)

zbermecasa

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Re: Grinding a 10" F/4 mirror for a first project
« Reply #15 on: January 31, 2018, 12:59:57 PM »
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It looks like a Ross null tester would be the way to go.......what do y'all think?

Would also like to build a nice Foucault tester too...


I have been trying to get the message across about star testing as the easiest and most accurate way to final figure mirrors. Suiter's book http://www.willbell.com/tm/tm5.htm is pretty much the definitive one on the theory and practice, but do not be daunted by its 428 pages, all a beginner needs are the 17 pages of Chapter 2 "An abbreviated star test manual".

Knife edge or Ronchi testing are also easy and good for the rough polishing and figuring, to check how the figure is progressing, but once you start getting close, the last few waves of parabolic correction, the star test is far easier to see the subtle defects.

BTW, the late John Dobson was an ardent supporter of star testing.

Mike

Christopher Buchanan

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Re: Grinding a 10" F/4 mirror for a first project
« Reply #16 on: February 02, 2018, 09:36:15 PM »
I might get flamed here but If you're going for a 10" f4 I would strongly recommend never starting the mirror making game using only full sized figuring tools. It may just be me but I can't get my head around how people figure a mirror with a full sized tool. Using smaller tool make much more sense and your methods when sorted can then be applied to a mirror of any size because the same rules will apply.

When I refigured my Royce 10" f4 I used only a 4" tool with all the same methods as I did for my 18" and 20" mirrors and it turned out great.

ovhercayvic

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Re: Grinding a 10" F/4 mirror for a first project
« Reply #17 on: February 09, 2018, 04:55:01 AM »
Mike- Good advice, I will def. give that a try.

Raymond - I agree. I have watched videos where the hogging and shaping was done with a 4"can with dental stone (cat food can), This made sense to me.....glad to hear someone else has experienced success with the method.